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Abstract:

There is provided an objective optical system including an optical element
having a phase shift structure, and a single-element objective lens made
of resin, wherein the phase shift structure includes a plurality of
refractive surface zones, the phase shift structure includes a first area
to contribute to converging at least the third light beam on a record
surface of the third optical disc, the first area includes at least two
types of steps, each of which is formed at a boundary between adjacent
ones of the plurality of refractive surface zones, the at least two types
of steps gives optical path length differences different from each other
to an incident light beam, the annular zone structure satisfies following
conditions:
0.01<(EP21-EP11)/EP11<0.10
0.04<(EP31-EP11)/EP11<0.30
-100<Σ(ΔOPD11/λ1)+Σ(ΔOPD12/λ.s-
ub.1)<-10
where EP11=INT((ΔOPD11/λ1)+0.5)×(λ1(n1-1-
)),
EP21=INT((ΔOPD21/λ2)+0.5)×(λ2(n1-1)),
EP31=INT((ΔOPD31/λ3)+0.5)×(λ3(n1-1)).

Claims:

1. An objective optical system used for an optical information
recording/reproducing device for recording information to and/or
reproducing information from at least three types of optical discs, by
selectively using one of three types of substantially collimated light
beams including a first light beam having a first wavelength
λ1 (nm), a second light beam having a second wavelength
λ2 (nm) and a third light beam having a third wavelength
λ3 (nm),the at least three types of optical discs including a
first optical disc for which information recording or information
reproducing is executed by using the first light beam, a second optical
disc for which information recording or information reproducing is
executed by using the second light beam, and a third optical disc for
which information recording or information reproducing is executed by
using the third light beam,the first, second and third wavelengths
λ1, λ2 and λ3 satisfying a
condition:λ1<λ2<λ3,when protective
layer thicknesses of the first, second and third optical discs are
represented by t1 (mm), t2 (mm) and t3 (mm), respectively, the protective
layer thicknesses satisfying a condition of t1<t2<t3,when numerical
apertures required for information reproducing or information recording
on the first, second and third optical discs are defined as NA1, NA2 and
NA3, respectively, the numerical apertures satisfying following
relationships:(NA1>NA3); and(NA2>NA3),the objective optical system
comprising:an optical element configured to have a phase shift structure
on at least one surface of the optical element; anda single-element
objective lens made of resin located between the optical element and an
optical disc being used,the phase shift structure including a plurality
of refractive surface zones concentrically formed about a predetermined
axis,the phase shift structure including a first area to contribute to
converging at least the third light beam on a record surface of the third
optical disc,the first area including at least two types of steps, each
of which is formed at a boundary between adjacent ones of the plurality
of refractive surface zones,the at least two types of steps giving
optical path length differences different from each other to an incident
light beam,when m11 represents a diffraction order at which diffraction
efficiency for the first light beam given by a first step of the at least
two types of steps in the first area is maximized, m21 represents a
diffraction order at which diffraction efficiency for the second light
beam given by the first step is maximized, m31 represents a diffraction
order at which diffraction efficiency for the third light beam given by
the first step is maximized, m12 represents a diffraction order at which
diffraction efficiency for the first light beam given by a second step of
the at least two types of steps in the first area is maximized, n1
represents a refractive index of the optical element with respect to the
first light beam, n2 represents a refractive index of the optical element
with respect to the second light beam, and n3 represents a refractive
index of the optical element with respect to the third light beam, the
phase shift structure satisfying following
conditions:0.01<(E21-E11)/E11<0.10
(2);0.04<(E31-E11)/E11<0.30 (3); and-100<φ1+φ2<-10
(4),where
E11=m11(λ1/(n1-1)),E21=m21(λ2/(n2-1)),E31=m31(.lamd-
a.3/(n3-1)),φ1=ΣP12ih2i×m11 (unit:
λ1),φ2=ΣP22ih2i×m12 (unit:
λ1),P12i (i: natural number) represents a 2i-order
coefficient of an optical path difference function defining the first
step, and P22i represents a 2i-order coefficient of an optical path
difference function defining the second step.

3. The objective optical system according to claim 1,wherein the optical
element is configured such that, with regard to the first light beam, a
refracting effect is cancelled by an effect of giving an optical path
length difference by the phase shift structure so that the optical
element has almost no power with respect to the first light beam,wherein
the optical element has Abbe number νd satisfying a
condition:15<νd<40 (1),wherein the phase shift structure takes
values of m11=10, m21=6 and m31=5.

4. The objective optical system according to claim 1,wherein:the phase
shift structure includes three types of steps, each of which is formed at
a boundary between adjacent ones of the plurality of refractive surface
zones;at least one type of the three types of steps is configured such
that a diffraction order at which diffraction efficiency for the first
light beam is maximized is a second order, a diffraction order at which
diffraction efficiency for the second light beam is maximized is a first
order, and a diffraction order at which diffraction efficiency for the
third light beam is maximized is a first order.

5. The objective optical system according to claim 1,whereinthe phase
shift structure includes a second area located outside the first area;the
second area is configured to contribute to converging the first and
second light beams on record surfaces of the first and second optical
discs, respectively, and not to contribute to convergence of the third
light beam;the second area includes a step -formed at a boundary between
adjacent ones of the plurality of refractive surface zones, the step in
the second area giving at least one type of optical path length
difference to an incident light beam;an absolute value of the at least
one type of optical path length difference given by the step in the
second area is approximately equal to an odd multiple of the first
wavelength of the first light beam.

6. The objective optical system according to claim 5,wherein the absolute
value of the at least one type of optical path length difference given by
the step in the second area is approximately equal to 3.lamda..sub.1.

7. The objective optical system according to claim 5,wherein the absolute
value of the at least one type of optical path length difference given by
the step in the second area is approximately equal to 5.lamda..sub.1.

8. The objective optical system according to claim 5,wherein:the phase
shift structure includes a third area located outside the second area;the
third area is configured to contribute to converging the first light beam
on the record surface of the first optical disc, and not to contribute to
convergence of each of the second and third light beams;the third area
includes a step formed at a boundary between adjacent ones of the
plurality of refractive surface zones, the step in the third area giving
at least one type of optical path length difference to an incident light
beam; andan absolute value of the at least one type of optical path
length difference given by the step in the third area is different from
absolute values of all types of optical path length differences given by
the second area.

9. The objective optical system according to claim 8, wherein the at least
one type of optical path length difference given by the step in the third
area is approximately equal to 1.lamda..sub.1.

10. An optical information recording/reproducing device for recording
information to and/or reproducing information from at least three types
of optical discs, by selectively using one of three types of
substantially collimated light beams including a first light beam having
a first wavelength λ1 (nm), a second light beam having a
second wavelength λ2 (nm) and a third light beam having a
third wavelength λ3 (nm),the at least three types of optical
discs including a first optical disc for which information recording or
information reproducing is executed by using the first light beam, a
second optical disc for which information recording or information
reproducing is executed by using the second light beam, and a third
optical disc for which information recording or information reproducing
is executed by using the third light beam,the first, second and third
wavelengths λ1, λ2 and λ3 satisfying a
condition:λ1<λ2<λ3,when protective
layer thicknesses of the first, second and third optical discs are
represented by t1 (mm), t2 (mm) and t3 (mm), respectively, the protective
layer thicknesses satisfying condition of t1<t2<t3,when numerical
apertures required for information reproducing or information recording
on the first, second and third optical discs are defined as NA1, NA2 and
NA3, respectively, the numerical apertures satisfying following
relationships:(NA1>NA3); and(NA2>NA3),the optical information
recording/reproducing device comprising:light sources respectively
emitting the first to third light beams;conversion optical components
respectively converging the first to third light beams into collimated
light beams; andan objective optical system,the objective optical system
comprising:an optical element configured to have a phase shift structure
on at least one surface of the optical element; anda single-element
objective lens made of resin located between the optical element and an
optical disc being used,the phase shift structure including a plurality
of refractive surface zones concentrically formed about a predetermined
axis,the phase shift structure including a first area to contribute to
converging at least the third light beam on a record surface of the third
optical disc,the first area including at least two types of steps, each
of which is formed at a boundary between adjacent ones of the plurality
of refractive surface zones,the at least two types of steps giving
optical path length differences different from each other to an incident
light beam,the protective layer thicknesses of the first to third optical
discs being defined as t3-t1.gtoreq.1.0 mm, and t2.apprxeq.0.6 mm,when
m11 represents a diffraction order at which diffraction efficiency for
the first light beam given by a first step of the at least two types of
steps in the first area is maximized, m21 represents a diffraction order
at which diffraction efficiency for the second light beam given by the
first step is maximized, m31 represents a diffraction order at which
diffraction efficiency for the third light beam given by the first step
is maximized, m12 represents a diffraction order at which diffraction
efficiency for the first light beam given by a second step of the at
least two types of steps in the first area is maximized, n1 represents a
refractive index of the optical element with respect to the first light
beam, n2 represents a refractive index of the optical element with
respect to the second light beam, and n3 represents a refractive index of
the optical element with respect to the third light beam, the phase shift
structure satisfying following conditions:0.01<(E21-E11)/E11<0.10
(2);0.04<(E31-E11)/E11<0.30 (3); and-100<φ1+φ2<-10
(4),where
E11=m11(λ1/(n1-1)),E21=m21(λ2/(n2-1)),E31=m31(.lamd-
a.3/(n3-1)),φ1=ΣP12ih2i×m11 (unit:
λ1),φ2=ΣP22ih2i×m12 (unit:
λ1),P12i (i: integer) represents a 2i-order coefficient
of an optical path difference function defining the first step, and
P22i represents a 2i-order coefficient of an optical path difference
function defining the second step.

11. An objective optical system used for an optical information
recording/reproducing device for recording information to and/or
reproducing information from at least three types of optical discs, by
selectively using one of three types of substantially collimated light
beams including a first light beam having a first wavelength
λ1 (nm), a second light beam having a second wavelength
λ2 (nm) and a third light beam having a third wavelength
λ3 (nm),the at least three types of optical discs including a
first optical disc for which information recording or information
reproducing is executed by using the first light beam, a second optical
disc for which information recording or information reproducing is
executed by using the second light beam, and a third optical disc for
which information recording or information reproducing is executed by
using the third light beam,the first, second and third wavelengths
λ1, λ2 and λ3 satisfying a
condition:λ1<λ2<λ3,when protective
layer thicknesses of the first, second and third optical discs are
represented by t1 (mm), t2 (mm) and t3 (mm), respectively, the protective
layer thicknesses satisfying a condition of t1 <t2 <t3,when
numerical apertures required for information reproducing or information
recording on the first, second and third optical discs are defined as
NA1, NA2 and NA3, respectively, the numerical apertures satisfying
following relationships:(NA1>NA3); and(NA2>NA3),the objective
optical system comprising:an optical element configured to have a phase
shift structure on at least one surface of the optical element; anda
single-element objective lens made of resin located between the optical
element and an optical disc being used,the phase shift structure
including a plurality of refractive surface zones concentrically formed
about a predetermined axis,the phase shift structure including a first
area to contribute to converging at least the third light beam on a
record surface of the third optical disc,the first area including at
least two types of steps, each of which is formed at a boundary between
adjacent ones of the plurality of refractive surface zones,the at least
two types of steps giving optical path length differences different from
each other to an incident light beam,the annular zone structure
satisfying following conditions:0.01<(EP21-EP11)/EP11<0.10
(7);0.04<(EP31-EP11)/EP11<0.30 (8);
and-100<Σ(ΔOPD11/λ1)+Σ(ΔOPD12/.lamd-
a.1)<-10 (9),where
EP11=INT((ΔOPD11/λ1)+0.5)×(λ2(n1-1)),EP-
21=INT((ΔOPD21/λ2)+0.5)×(λ2(n1-1)),EP31=-
INT((ΔOPD31/λ3)+0.5)×(λ3(n1-1)),ΔO-
PD11/λ1 denotes an optical path length difference given by a
first step of the at least two types of steps in the first area to the
first light beam, ΔOPD21/λ2 denotes an optical path
length difference given by the first step to the second light beam, and
ΔOPD31/λ3 denotes an optical path length difference
given by the first step to the third light beam, and
ΔOPD12/λ1 denotes an optical path length difference
given by a second step of the at least two types of steps to the first
light beam, n1 represents a refractive index of the optical element with
respect to the first light beam, n2 represents a refractive index of the
optical element with respect to the second light beam, and n3 represents
a refractive index of the optical element with respect to the third light
beam.

13. The objective optical system according to claim 11,wherein the optical
element is configured such that, with regard to the first light beam, a
refracting effect is cancelled by an effect of giving an optical path
length difference by the phase shift structure so that the optical
element has almost no power with respect to the first light beam,wherein
the optical element has Abbe number νd satisfying a
condition:15<νd<40 (1),wherein one of the at least two types
of steps satisfies a
condition:9.85<|ΔOPD11/λ1|<10.35 (12).

14. The objective optical system according to claim 11,wherein:the phase
shift structure includes three types of steps giving optical path length
differences to an incident beam, each of the three types of steps being
formed at a boundary between adjacent ones of the plurality of refractive
surface zones; andat least one type of the three types of steps gives an
optical path length difference, an absolute value of which is
approximately equal to 2.lamda.1 to the first light beam.

15. The objective optical system according to claim 11,whereinthe phase
shift structure includes a second area located outside the first area;the
second area is configured to contribute to converging the first and
second light beams on record surfaces of the first and second optical
discs, respectively, and not to contribute to convergence of the third
light beam;the second area includes a step formed at a boundary between
adjacent ones of the plurality of refractive surface zones, the step in
the second area giving at least one type of optical path length
difference to an incident light beam; andan absolute value of the at
least one type of optical path length difference given by the step in the
second area is approximately equal to an odd multiple of the first
wavelength of the first light beam.

16. The objective optical system according to claim 15,wherein the
absolute value of the at least one type of optical path length difference
given by the step in the second area is approximately equal to
3.lamda..sub.1.

17. The objective optical system according to claim 15,wherein the
absolute value of the at least one type of optical path length difference
given by the step in the second area is approximately equal to
5.lamda..sub.1.

18. The objective optical system according to claim 15,wherein:the phase
shift structure includes a third area located outside the second area;the
third area is configured to contribute to converging the first light beam
on the record surface of the first optical disc, and not to contribute to
convergence of each of the second and third light beams;the third area
includes a step formed at a boundary between adjacent ones of the
plurality of refractive surface zones, the step in the third area giving
at least one type of optical path length difference to an incident light
beam;an absolute value of the at least one type of optical path length
difference given by the step in the third area is different from absolute
values of all types of optical path length differences given by the
second area.

19. The objective optical system according to claim 18, wherein the at
least one type of optical path length difference given by the step in the
third area is approximately equal to 1.lamda..sub.1.

20. An optical information recording/reproducing device for recording
information to and/or reproducing information from at least three types
of optical discs, by selectively using one of three types of
substantially collimated light beams including a first light beam having
a first wavelength λ1 (nm), a second light beam having a
second wavelength λ2 (nm) and a third light beam having a
third wavelength λ3 (nm),the at least three types of optical
discs including a first optical disc for which information recording or
information reproducing is executed by using the first light beam, a
second optical disc for which information recording or information
reproducing is executed by using the second light beam, and a third
optical disc for which information recording or information reproducing
is executed by using the third light beam,the first, second and third
wavelengths λ1, λ2 and λ3 satisfying a
condition:λ1<λ2<λ3,when protective
layer thicknesses of the first, second and third optical discs are
represented by t1 (mm), t2 (mm) and t3 (mm), respectively, the protective
layer thicknesses satisfying a condition of t1<t2<t3,when numerical
apertures required for information reproducing or information recording
on the first, second and third optical discs are defined as NA1, NA2 and
NA3, respectively, the numerical apertures satisfying following
relationships:(NA1>NA3); and(NA2>NA3),the optical information
recording/reproducing device comprising:light sources respectively
emitting the first to third light beams;conversion optical components
respectively converging the first to third light beams into collimated
light beams; andan objective optical system,the objective optical system
comprising:an optical element configured to have a phase shift structure
on at least one surface of the optical element; anda single-element
objective lens made of resin located between the optical element and an
optical disc being used,the phase shift structure including a plurality
of refractive surface zones concentrically formed about a predetermined
axis,the phase shift structure including a first area to contribute to
converging at least the third light beam on a record surface of the third
optical disc,the first area including at least two types of steps, each
of which is formed at a boundary between adjacent ones of the plurality
of refractive surface zones,the at least two types of steps giving
optical path length differences different from each other to an incident
light beam,the protective layer thicknesses of the first to third optical
discs being defined as t3-t1.gtoreq.1.0 mm, and t2.apprxeq.0.6 mm,the
annular zone structure satisfying following
conditions:0.01<(EP21-EP11)/EP11<0.10
(7);0.04<(EP31-EP11)/EP11<0.30 (8);
and-100<Σ(ΔOPD11/λ1)+Σ(ΔOPD12/.lamd-
a.1)<-10 (9),where
EP11=INT((ΔOPD11/λ1)+0.5)×(λ1(n1-1)),EP-
21=INT((ΔOPD21/λ2)+0.5)×(λ2(n1-1)),EP31=-
INT((ΔOPD31/λ3)+0.5)×(λ3(n1-1)),ΔO-
PD11/λ1 denotes an optical path length difference given by a
first step of the at least two types of steps in the first area to the
first light beam, ΔOPD21/λ2 denotes an optical path
length difference given by the first step to the second light beam, and
ΔOPD31/λ3 denotes an optical path length difference
given by the first step to the third light beam, and
ΔOPD12/λ1 denotes an optical path length difference
given by a second step of the at least two types of steps to the first
light beam, n1 represents a refractive index of the optical element with
respect to the first light beam, n2 represents a refractive index of the
optical element with respect to the second light beam, and n3 represents
a refractive index of the optical element with respect to the third light
beam.

Description:

BACKGROUND OF THE INVENTION

[0001]The present invention relates to an objective optical system which
is installed in a device employing multiple types of light beams having
different wavelengths, such as an optical information
recording/reproducing device for recording information to and/or
reproducing information from multiple types of optical discs differing in
recording density.

[0002]There exist various standards of optical discs (CD, DVD, etc.)
differing in recording density, protective layer thickness, etc.
Meanwhile, new-standard optical discs (HD DVD (High-Definition DVD), BD
(Blu-ray Disc), etc.), having still higher recording density than DVD,
are being brought into practical use in recent years to realize still
higher information storage capacity. The protective layer thickness of
such a new-standard optical disc is substantially equal to or less than
that of DVD. In consideration of user convenience with such optical discs
according to multiple standards, the optical information
recording/reproducing devices (more specifically, objective optical
systems installed in the devices) of recent years are required to have
compatibility with the above three types of optical discs. Incidentally,
in this specification, the "optical information recording/reproducing
devices" include devices for both information reproducing and information
recording, devices exclusively for information reproducing, and devices
exclusively for information recording. The above "compatibility" means
that the optical information recording/reproducing device ensures the
information reproducing and/or information recording with no need of
component replacement even when the optical disc being used is switched.

[0003]In order to provide an optical information recording/reproducing
device having the compatibility with optical discs of multiple standards,
the device has to be configured to be capable of forming a beam spot
suitable for a particular recording density of an optical disc being
used, by changing a NA (Numerical Aperture) of an objective optical
system used for information reproducing/registering, while also
correcting spherical aberration which varies depending on the protective
layer thickness changed by switching between optical discs of different
standards. Since the diameter of the beam spot can generally be made
smaller as the wavelength of the beam gets shorter, multiple laser beams
having different wavelengths are selectively used by the optical
information recording/reproducing device depending on the recording
density of the optical disc being used. For example, for CDs, a laser
beam with a wavelength of approximately 790 nm (a so-called near-infrared
laser) is used. For DVDs, a laser beam with a wavelength of approximately
660 nm (a so-called red laser) shorter than the wavelength for CDs is
used. For the aforementioned new-standard optical discs, a laser beam
with a wavelength still shorter than that for DVDs (e.g., so-called "blue
laser" around 408 nm) is used in order to deal with the extra-high
recording density.

[0004]Examples of an optical system for suitably converging the laser
beams on the three types of optical discs, respectively, are disclosed in
Japanese Patent Provisional Publications Nos. 2006-164498A (hereafter,
referred to as JP2006-164498A), 2006-12394A (hereafter, referred to as
JP2006-12394A), 2007-122828A (hereafter, referred to as JP2007-122828A)
and 2005-158217A (hereafter, referred to as JP2005-158217A).

[0005]An objective optical system disclosed in Japanese Patent Provisional
Publication No. 2006-164498A (hereafter, referred to as JP2006-164498A)
is configured such that at least one surface of an objective lens or an
at least one surface of an optical element located on the front side of
the objective lens is provided with a diffraction surface. The
diffraction surface is configured such that the diffraction order at
which the diffraction efficiency for the blue laser beam is maximized is
an even order. Each of the blue laser and the red laser is incident on
the objective optical system as a collimated beam, and the near-infrared
laser beam is incident on the objective optical system as a
non-collimated beam (a diverging beam). As described above, the objective
optical system disclosed in JP2006-164498A has the compatibility with the
plurality of types of optical discs of different standards by
appropriately selecting the diffraction effect and the degree of
divergence for each of the plurality of types of optical discs.

[0006]An objective optical system disclosed in JP2006-12394A is configured
to have an optical element (or an objective lens) formed by cementing two
types of optical components made of different materials with respect to
each other. A diffraction structure is formed on a cementing surface
between the two types of components. The objective optical system is
designed so that, through use of the difference between the refractive
indexes of the two types of optical components and the diffraction
effect; the optical element can enhance the use efficiency for each of
the different types of laser beams.

[0007]In an optical system disclosed in JP2007-122828A, substantially the
same optical configuration as that disclosed in JP2006-12394A is employed
to maintain the diffraction efficiency at a high level for each of the
blue laser and the near-infrared laser. More specifically, JP2007-122828A
discloses an optical pick-up device configured to have a diffraction
grating formed by laminating at least two types of elements having
different degrees of dispersion together so that high diffraction
efficiency can be maintained for both of the blue laser and the infrared
laser. JP2007-122828A also discloses an optical pick-up device provided
with an optical element having a single diffraction surface designed to
appropriately select, for each of the blue laser and the near-infrared
laser, the diffraction order at which the diffraction efficiency is
maximized.

[0008]JP2005-158217A discloses an objective optical system for an optical
pick-up provided with a diffraction optical element located on the front
side of the objective lens. More specifically, the diffraction optical
element includes a diffraction surface which has no diffraction effect on
the blue laser and the near-infrared laser but has diffraction effect on
the red laser, and a diffraction surface which has no diffraction effect
on the blue laser and the red laser but has diffraction effect on the
near-infrared laser. By thus employing the diffraction optical element
having two diffraction surfaces with different diffraction effects, the
optical pick-up achieves the compatibility with the different types of
optical discs.

[0009]However, the optical configurations disclosed in JP2006-164498A,
JP2006-12394A, JP2007-122828A and JP2005-158217A have the following
drawbacks.

[0010]Since the diffraction structure of the objective optical system
disclosed in JP2006-164498A is configured such that the diffraction order
at which the diffraction efficiency for the blue laser is maximized is an
even order, it is necessary to use a non-collimated beam for at least one
of the plurality of types of laser beams. If a non-collimated beam is
used, off-axis aberration, such as a soma, is inevitably caused when the
objective lens shifts in a plane perpendicular to an optical axis of the
objective lens, fox example, during a tracking operation.

[0011]In manufacturing the objective optical system disclosed in
JP2006-12394A, the manufacturing process increases for a cementing
process. In addition, it is necessary to appropriately from the
diffraction structure on the cementing surface. Therefore, the
manufacturing of the objective optical system requires considerably high
accuracy, which increases the manufacturing cost.

[0012]Since the optical system disclosed in JP2007-122828A uses the
diffraction grating, the manufacturing of the optical system requires the
considerably high accuracy, and the manufacturing cost is increased. The
diffraction surface of the optical element disclosed in JP2007-122828A is
configured to produce the intense diffraction light of an even order for
the blue laser. In this case, it is difficult to correct the relative
spherical aberration caused by switching between an optical disc
requiring the blue laser and an optical disc requiring the near-infrared
laser.

[0013]Since the diffraction optical element disclosed in JP2005-158217A
does not have the diffraction effect on the blue laser, it is impossible
to control the spherical aberration caused by the wavelength variations
and the temperature variations. In particular, if resin is used as
material of the objective lens, the spherical aberration caused due to
the temperature variations becomes considerably large. Therefore, in this
case, a spherical aberration correction element (e.g., a liquid crystal
element) having a complicated structure is required.

SUMMARY OF THE INVENTION

[0014]The present invention is advantageous in that it provides at least
one of an objective optical system and an optical information
recording/reproducing device configured to have compatibility with
multiple types of optical discs of different standards, to maintain the
use efficiency of light for optical discs (e.g., BD) having high
recording densities at a high level while increasing the use efficiency
of light for other optical discs having relatively low recording
densities, and to be manufactured easily at a low cost.

[0015]According to an aspect of the invention, there is provided an
objective optical system used for an optical information
recording/reproducing device for recording information to and/or
reproducing information from at least three types of optical discs, by
selectively using one of three types of substantially collimated light
beams including a first light beam having a first wavelength
λ1 (nm), a second light beam having a second wavelength
λ2 (nm) and a third light beam having a third wavelength
λ3 (nm), The at least three types of optical discs includes a
first optical disc for which information recording or information
reproducing is executed by using the first light beam, a second optical
disc for which information recording or information reproducing is
executed by using the second light beam, and a third optical disc for
which information recording or information reproducing is executed by
using the third light beam. The first, second and third wavelengths
λ1, λ2 and λ3 satisfies a condition
λ1<λ2<λ3. When protective layer
thicknesses of the first, second and third optical discs are represented
by t1 (mm), t2 (mm) and t3 (mm), respectively, the protective layer
thicknesses satisfy a condition of t1<t2<t3. When numerical
apertures required for information reproducing or information recording
on the first, second and third optical discs are defined as NA1, NA2 and
NA3, respectively, the numerical apertures satisfy following
relationships: (NA1>NA3); and (NA2>NA3).

[0016]In this configuration, the objective optical system includes an
optical element configured to have a phase shift structure on at least
one surface of the optical element; and a single-element objective lens
made of resin located between the optical element and an optical disc
being used. The phase shift structure includes a plurality of refractive
surface zones concentrically formed about a predetermined axis. The phase
shift structure includes a first area to contribute to converging at
least the third light beam on a record surface of the third optical disc.
The first area includes at least two types of steps, each of which is
formed at a boundary between adjacent ones of the plurality of refractive
surface zones. The at least two types of steps gives optical path length
differences different from each other to an incident light beam.

[0017]When m11 represents a diffraction order at which diffraction
efficiency for the first light beam given by a first step of the at least
two types of steps in the first area is maximized, m21 represents a
diffraction order at which diffraction efficiency for the second light
beam given by the first step is maximized, m31 represents a diffraction
order at which diffraction efficiency for the third light beam given by
the first step is maximized, m12 represents a diffraction order at which
diffraction efficiency for the first light beam given by a second step of
the at least two types of steps in the first area is maximized, n1
represents a refractive index of the optical element with respect to the
first light beam, n2 represents a refractive index of the optical element
with respect to the second light beam, and n3 represents a refractive
index of the optical element with respect to the third light beam, the
phase shift structure satisfies following conditions:

0.01<(E21-E11)/E11<0.10 (2);

0.04<(E31-E11)/E11<0.30 (3); and

-100<φ1+φ2<-10 (4),

where E11=m11(λ1/(n1-1)),

E21=m21(λ2/(n2-1)),

E31=m31(λ3/(n3-1)),

φ1=ΣP12ih2i×m11 (unit: λ1),

φ2=ΣP22ih2i×m12 (unit: λ1),

[0018]P12i (i: natural number) represents a 2i-order coefficient of
an optical path difference function defining the first step, and
P22i represents a 2i-order coefficient of an optical path difference
function defining the second step.

[0019]Such a configuration makes it possible to achieve relatively high
use efficiency of light for each of the light beams while suppressing the
spherical aberration for information recording or information reproducing
of each of the three types of optical discs.

[0021]In at least one aspect, the optical element is configured such that,
with regard to the first light beam, a refracting effect is cancelled by
an effect of giving an optical path length difference by the phase shift
structure so that the optical element has almost no power with respect to
the first light beam,

[0022]wherein the optical element has Abbe number νd satisfying a
condition:

[0024]In at least one aspect, the phase shift structure includes three
types of steps, each of which is formed at a boundary between adjacent
ones of the plurality of refractive surface zones. At least one type of
the three types of steps is configured such that a diffraction order at
which diffraction efficiency for the first light beam is maximized is a
second order, a diffraction order at which diffraction efficiency for the
second light beam is maximized is a first order, and a diffraction order
at which diffraction efficiency for the third light beam is maximized is
a first order.

[0025]According to another aspect of the invention, there is provided an
objective optical system used for an optical information
recording/reproducing device for recording information to and/or
reproducing information from at least three types of optical discs, by
selectively using one of three types of substantially collimated light
beams including a first light beam having a first wavelength
λ1 (nm), a second light beam having a second wavelength
λ2 (nm) and a third light beam having a third wavelength
λ3 (nm). The at least three types of optical discs includes a
first optical disc for which information recording or information
reproducing is executed by using the first light beam, a second optical
disc for which information recording or information reproducing is
executed by using the second light beam, and a third optical disc for
which information recording or information reproducing is executed by
using the third light beam. The first, second and third wavelengths
λ1, λ2 and λ3 satisfies a condition
λ1<λ2<λ3. When protective layer
thicknesses of the first, second and third optical discs are represented
by t1 (mm), t2 (mm) and t3 (mm), respectively, the protective layer
thicknesses satisfy a condition of t1<t2<t3. When numerical
apertures required for information reproducing or information recording
on the first, second and third optical discs are defined as NA1, NA2 and
NA3, respectively, the numerical apertures satisfy following
relationships: (NA1>NA3); and (NA2>NA3).

[0026]In this configuration, The objective optical system includes an
optical element configured to have a phase shift structure on at least
one surface of the optical element, and a single-element objective lens
made of resin located between the optical element and an optical disc
being used. The phase shift structure includes a plurality of refractive
surface zones concentrically formed about a predetermined axis. The phase
shift structure includes a first area to contribute to converging at
least the third light beam on a record surface of the third optical disc.
The first area includes at least two types of steps, each of which is
formed at a boundary between adjacent ones of the plurality of refractive
surface zones. The at least two types of steps gives optical path length
differences different from each other to an incident light beam. The
annular zone structure satisfies following conditions:

0.01<(EP21-EP11)/EP11<0.10 (7);

0.04<(EP31-EP11)/EP11<0.30 (8); and

-100<Σ(ΔOPD11/λ1)+Σ(ΔOPD12/λ.s-
ub.1)<-10 (9),

where EP11=INT((ΔOPD11/λ1)+0.5)×(λ1(n1-1-
)),

EP21=INT((ΔOPD21/λ2)+0.5)×(λ2(n1-1)),

EP31=INT((ΔOPD31/λ3)+0.5)×(λ3(n3-1)),

[0027]ΔOPD11/λ1, denotes an optical path length
difference given by a first step of the at least two types of steps in
the first area to the first light beam, ΔOPD21/λ2
denotes an optical path length difference given by the first step to the
second light beam, and ΔOPD31/λ3 denotes an optical path
length difference given by the first step to the third light beam, and
ΔOPD12/λ1 denotes an optical path length difference
given by a second step of the at least two types of steps to the first
light beam, n1 represents a refractive index of the optical element with
respect to the first light beam, n2 represents a refractive index of the
optical element with respect to the second light beam, and n3 represents
a refractive index of the optical element with respect to the third light
beam.

[0028]Such a configuration makes it possible to achieve relatively high
use efficiency of light for each of the light beams while suppressing the
spherical aberration for information recording or information reproducing
of each of the three types of optical discs.

[0030]In at least one aspect, the optical element is configured such that,
with regard to the first light beam, a refracting effect is cancelled by
an effect of giving an optical path length difference by the phase shift
structure so that the optical element has almost no power with respect to
the first light beam. The optical element has Abbe number νd
satisfying a condition:

15<νd<40 (1).

Further, one of the at least two types of steps satisfies a condition:

9.85<|ΔOPD11/λ1|<10.35 (12).

[0031]In at least one aspect, the phase shift structure includes three
types of steps giving optical path length differences to an incident
beam, each of the three types of steps being formed at a boundary between
adjacent ones of the plurality of refractive surface zones. At least one
type of the three types of steps gives an optical path length difference
approximately equal to 2λ1 to the first light beam.

[0032]With regard to the above described two aspects of the invention
concerning the objective optical system, the phase shift structure may
include a second area located outside the first area. In this case, the
second area is configured to contribute to converging the first and
second light beams on record surfaces of the first and second optical
discs, respectively, and not to contribute to convergence of the third
light beam. The second area includes a step formed at a boundary between
adjacent ones of the plurality of refractive surface zones, the step in
the second area giving at least one type of optical path length
difference to an incident light beam. An absolute value of the at least
one type of optical path length difference given by the step in the
second area is approximately equal to an odd multiple of the first
wavelength of the first light beam.

[0033]In at least one aspect, the absolute value of the at least one type
of optical path length difference given by the step in the second area is
approximately equal to 3λ1.

[0034]In at least one aspect, the absolute value of the at least one type
of optical path length difference given by the step in the second area is
approximately equal to 5λ1.

[0035]In at least one aspect, the phase shift structure includes a third
area located outside the second area. In this case, the third area is
configured to contribute to converging the first light beam on the record
surface of the first optical disc, and not to contribute to convergence
of each of the second and third light beams. The third area includes a
step formed at a boundary between adjacent ones of the plurality of
refractive surface zones, the step in the third area giving at least one
type of optical path length difference to an incident light beam. An
absolute value of the at least one type of optical path length difference
given by the step in the third area is different from absolute values of
all types of optical path length differences given by the second area.

[0036]In at least one aspect, the objective optical system according to
claim 16, wherein the at least one type of optical path length difference
given by the step in the third area is approximately equal to
1λ1.

[0037]According to another aspect of the invention, there is provided an
optical information recording/reproducing device for recording
information to and/or reproducing information from at least three types
of optical discs. The optical information recording/reproducing device
includes light sources respectively emitting the first to third light
beams, conversion optical components respectively converging the first to
third light beams into collimated light beams, and the above mentioned
objective optical system. In this configuration the protective layer
thicknesses of the first to third optical discs are defined as
t3-t1≧1.0 mm, and t2≈0.6 mm.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

[0038]FIG. 1 illustrates a general configuration of an optical information
recording/reproducing device on which an objective optical system is
mounted.

[0039]FIG. 2 is an optical block diagram of an objective optical system
according to a first example.

[0040]FIG. 3 is an optical block diagram of an objective optical system
according to a second example.

[0041]FIG. 4 is an optical block diagram of an objective optical system
according to a third example.

[0042]FIG. 5 is an optical block diagram of an objective optical system
according to a fourth example.

[0043]FIGS. 6A-6C show the spherical aberration caused in the objective
optical system according to the first example.

[0044]FIGS. 7A-7C show the spherical aberration caused in the objective
optical system according to the first example.

[0045]FIGS. 8A-8C show the spherical aberration caused in the objective
optical system according to the first example.

[0046]FIGS. 9A-9C show the spherical aberration caused in the objective
optical system according to the first example.

[0047]FIG. 10A is a front view illustrating a annular zone structure
formed on a first surface of an optical element, and FIG. 10B is a cross
sectional view of the optical element illustrating the annular zone
structure formed thereon.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0048]Hereinafter, an embodiment according to the invention are described
with reference to the accompanying drawings.

[0049]In the following, an objective optical system 30 according to the
embodiment, and an optical information recording/reproducing device 100
on which the objective optical system 30 is mounted are described (see
FIG. 1).

[0050]In the following explanation, an optical disc of a type (one of the
three types) having the highest recording density (e.g. a new-standard
optical disc such as BD) will be referred to as an "optical disc D1", an
optical disc of a type having a relatively low recording density compared
to the optical disc D1 (DVD, DVD-R, etc.) will be referred to as an
"optical disc D2", and an optical disc of a type having the lowest
recording density (CD, CD-R, etc.) will be referred to as an "optical
disc D3" for convenience of explanation.

[0051]If the protective layer thicknesses of the optical discs D1-D3 are
defined as t1, t2, t3, respectively, the protective layer thicknesses
satisfies the following relationship.

t1<t2<t3

[0052]In order to carry out the information reproducing/recording on each
of the optical discs D1-D3, the NA (Numerical Aperture) required for the
information reproducing/recording has to be varied properly so that a
beam spot suitable for the particular recording density of each optical
disc can be formed. When the optimum design numerical apertures required
for the information reproducing/recording on the three types of optical
discs D1, D2 and D3 are defined as NA1, NA2 and NA3, respectively, the
numerical apertures (NA1, NA2, NA3) satisfy the following relationships.

(NA1>NA3) and (NA2>NA3)

[0053]Specifically, for the information recording/reproducing on the
optical discs D1 and D2 having high recording densities, a relatively
large NA is required since a relatively small beam spot has to be formed.
On the other hand, for the information recording/reproducing on the
optical disc D3 having the lowest recording density, the required NA is
relatively small. Incidentally, each optical disc is set on a turntable
(not shown) and rotated at high speed when the information
recording/reproducing is carried out.

[0054]In cases where three types of optical discs D1-D3 (having different
recording densities) are used as above, multiple laser beams having
different wavelengths are selectively used by the optical information
recording/reproducing device so that a beam spot suitable for each
recording density can be formed on the record surface of the optical disc
being used.

[0055]Specifically, for the information recording/reproducing on the
optical disc D1, a "first laser beam" having the shortest wavelength is
emitted from a light source so as to form the smallest beam spot on the
record surface of the optical disc D1. On the other hand, for the
information recording/reproducing on the optical disc D3, a "third laser
beam" having the longest wavelength is emitted from a light source so as
to form the largest beam spot on the record surface of the optical disc
D3. For the information recording/reproducing on the optical disc D2, a
"second laser beam" having a wavelength longer than that of the first
laser beam and shorter than that of the third laser beam is emitted from
a light source so as to form a relatively small beam spot on the record
surface of the optical disc D2.

[0056]If the wavelengths of the first, second and third laser beams are
defined as λ1, λ2 and λ3, respectively,
the wavelengths satisfy the following relationship.

λ1<λ2<λ3

[0057]FIG. 1 illustrates a general configuration of the optical
information recording/reproducing device 100 on which the objective
optical system 30 is mounted. As shown in FIG. 1, the optical information
recording/reproducing device 100 includes a light source 1A which emits
the first laser beam, a light source 1B which emits the second laser
beam, a light source 1C which emits the third laser beam, diffraction
gratings 2A, 2B and 2C, coupling lenses 3A, 3B and 3C, beam splitters 41
and 42, half mirrors 5A, 5B and 5C, photoreceptors 6A, 6B and 6C, and the
objective optical system 30.

[0058]In FIG. 1, a reference axis AX of the optical information
recording/reproducing device 100 is indicated by a chain line. In a
normal state, an optical axis of the objective optical system 30
coincides with the reference axis AX of the optical information
recording/reproducing device 100. However, the optical axis of the
objective optical system 30 or an optical axis of an objective lens 20
may be shifted from the reference axis AX for a tracking operation.

[0059]As described above, the required NA varies depending on the type of
the optical disc being used. Therefore, the optical information
recording/reproducing device 100 may be provided with one or more
aperture stops for adjusting beam diameters of the first to third laser
beams.

[0060]Each optical disc has the protective layer and the record surface
(not shown). Practically, the record surface is sandwiched between the
protective layer and a substrate layer or a label layer.

[0061]As shown in FIG. 1, the first, second and third laser beams emitted
by the light sources 1A, 1B and 1C are directed to a common optical path
after passing through the diffraction gratings 2A, 2B, and 2C, the
coupling lenses 3A, 3B and 3C, and the beam splitters 41 and 42. Then,
each of the first, second and third laser beams enters the objective
optical system 30. The first, second and third laser beams emitted by the
light sources 1A, 1B and 1C are converted into collimated beams by the
coupling lenses 3A, 3B and 3C, respectively. That is, in this embodiment,
each of the coupling lenses 3A, 3B and 3C functions as a collimator lens.
Therefore, each of the first, second and third laser beams enters the
objective optical system 30 as a collimated beam.

[0062]By thus configuring the optical information recording/reproducing
device 100, it is possible to suitably suppress off-axis aberrations,
such as a coma, even if the objective optical system 30 (i.e., the
objective lens 20) shifts by a minute distance in a direction
perpendicular to the optical axis of the objective optical system 30 for
the tracking operation.

[0063]Each of the first, second and third laser beams passed through the
objective optical system 30 converges onto the record surface of the
corresponding optical disc. The laser beam reflected from the record
surface of each of the optical discs D1, D2 and D3 returns toward the
objective optical system 30 along the same optical path, and thereafter
passes through the corresponding one of the half mirror 5A, 5B and 5C
before finally detected by the corresponding one of the photoreceptors
6A, 6B and 6C.

[0064]Since the first to third laser beams having different wavelengths
are used for the optical discs D1-D3 in the optical information
recording/reproducing device 100, the spherical aberration varies
depending on change of the refractive index of the objective lens 10 and
the difference in protective layer thicknesses between the optical discs
D1-D3. In order to provide the compatibility with the three types of
optical discs D1-D3 for the optical information recording/reproducing
device 100, it is necessary to suitably correct the spherical aberration
for each of the optical discs D1-D3. In order to perform the information
recording/reproducing, for each of the optical discs D1-D3, in a high
degree of accuracy while keeping a high S/N level, it is necessary to
increase the use efficiency of light and thereby to use a sufficient
amount of light to form a beam spot having a predetermined diameter on
the record surface of the optical disc being used. For this reason, the
objective optical system 30 according to the embodiment is configured as
follows.

[0065]As shown in FIG. 1, the objective optical system 30 includes an
optical element 10 and the objective lens 20 arranged, along the optical
path, in this order from the light source side. FIG. 2 is an enlarged
view of the objective optical system 30. It should be noted that,
although in FIG. 2 the optical disc D1 is illustrated as an optical disc
being used, the objective optical system 30 provides the same
configuration as that shown in FIG. 2 for each of the optical discs D2
and D3.

[0066]As shown in FIG. 2, the optical element 10 has a first surface 11
and a second surface 12 arranged in this order from the light source
side. The objective lens 20 has a first surface 21 and a second surface
22 arranged in this order from the light source side. The objective lens
20 is a biconvex single-element lens.

[0067]Each of the first surface 11 of the optical element 10 and the first
and second surfaces 21 and 22 of the objective lens 20 is an aspherical
surface.

[0068]A shape of an aspherical surface is expressed by a following
equation:

[0069]where, X(h) represents a SAG amount which is a distance between a
point on the aspherical surface at a height of h from the optical axis
and a plane tangential to the aspherical surface at the optical axis,
symbol c represents curvature (l/r) on the optical axis, K is a conical
coefficient, and A2i (i: integer) represents an aspherical
coefficient of an even order larger than or equal to the fourth order. By
thus forming optical surfaces of the optical components of the objective
optical system 30 to be aspherical surfaces, it becomes possible to
suitably correct the spherical aberration.

[0070]The optical element 10 is made of a single material. In order to
secure easiness and effectiveness in manufacturing, the optical element
20 is made of resin. The material having the Abbe number νd satisfying
a following condition (1) is used as material of the optical element 10.

15<νd<40 (1)

[0071]As described above, the optical element 20 is made of material
having a relatively low Abbe number (i.e., material having a high degree
of dispersion). In addition, the optical element 10 is provided with an
annular zone structure. By this configuration, the objective optical
system enhances the use efficiency of light for all of the first to third
laser beams.

[0072]In general, a designer of an objective optical system for an optical
information recording/reproducing device tends to avoid use of material
having a high degree of dispersion because material having a high degree
of dispersion causes a relatively large amount of chromatic aberration.
By contrast, according to the embodiment, the optical element 10 is
configured such that, with regard to the first laser beam, the refracting
effect is cancelled by the effect of giving the optical path length
difference by the annular zone structure. In other words, the optical
element 10 has almost no power with respect to the first laser beam.
Consequently, the amount of chromatic aberration can be suppressed.

[0073]Such a configuration of the optical element 10 also makes it
possible to effectively suppress the aberrations caused when the
positional relationship between the optical element 10 and the objective
20 changes.

[0074]Hereafter, the annular zone structure formed on the optical element
10 is explained in detail. In this embodiment, at least one of first and
second surfaces 11 and 12 of the optical element 10 is provided with the
annular zone structure. The annular zone structure has a plurality of
refractive surface zones (annular zones) concentrically formed about the
reference axis AX. The plurality of annular zones are divided by minute
steps formed between adjacent ones of the plurality of annular zones to
extend in parallel with the optical axis of the optical element 10.

[0075]Each step is designed such that a predetermined optical path length
difference is caused between a laser beam passing through the inside of
the boundary and a laser beam passing through the outside of the
boundary. It is noted that such an annular zone structure may be called a
diffraction structure.

[0076]If the annular zone structure is designed such that the
predetermined optical path length difference is a n-fold value (n:
integer) of a particular wavelength α, the annular zone structure
may be expressed as an n-th order diffraction structure having the blazed
wavelength α. If a laser beam having a particular wavelength β
passes through the diffraction structure, the diffraction order having
the highest diffraction efficiency is equal to an integer "m" closest to
a value obtained by dividing the optical path length difference with the
wavelength β.

[0077]Considering the fact that a predetermined optical path length
difference is caused between the laser beam passing through the inside of
a boundary and the laser beam passing through the outside of the
boundary, the effect of the step can be regarded as shifting the phases
of the laser beam passing through the inside of a boundary and the laser
beam passing through the outside of the boundary with respect to each
other. In other words, the annular zone structure can be pressed as a
structure for phase-shifting an incident beam (i.e., a phase-shift
structure).

[0078]If the annular zone structure is considered as the diffraction
structure, the annular zone structure can be expressed by a following
optical path difference function φ(h). The optical path difference
function φ(h) represents the function as a diffraction lens in a form
of an additional optical path length at a height h from the optical axis.
That is, the optical path difference function φ(h) is a function
which defines the position and height of each step in the annular zone
structure (i.e., the diffraction structure).

[0079]More specifically, the optical path difference function φ(h) can
be expressed by an equation:

φ ( h ) = m λ i = 1 P 2 i
h 2 i ##EQU00002##

[0080]where P2i represents the 2i-th order coefficient (i: natural
number), h represents a height from the optical axis, m represents a
diffraction order at which the diffraction efficiency of the laser beam
being used is maximized, and λ represents a design wavelength of
the laser beam being used.

[0081]The annular zone structure provided on the optical element 10 is
defined not only by using a single optical path difference function but
also by combining a plurality of types of optical path difference
functions. In this embodiment, the annular zone structure includes two or
more types of steps giving different optical path length differences to
the incident beam. The two or more types of steps are obtained by
combining the plurality of types of optical path difference functions. By
this structure, it is possible to give a plurality of types of optical
effects to the incident beam.

[0082]In the following explanation, m11 represents the diffraction order
at which the diffraction efficiency for the first laser beam given by one
of the two or more steps (hereafter, referred to as a first step) is
maximized, m21 represents the diffraction order at which the diffraction
efficiency for the second laser beam given by the first step is
maximized, m31 represents the diffraction order at which the diffraction
efficiency for the third laser beam given by the first step is maximized,
m12 represents the diffraction order at which the diffraction efficiency
for the first laser beam given by another step of the two or more types
of steps (hereafter, referred to as a second step) giving an optical path
length difference different from the optical path length difference given
by the first step is maximized, m22 represents the diffraction order at
which the diffraction efficiency for the second laser beam given by the
second step is maximized, m32 represents the diffraction order at which
the diffraction efficiency for the third laser beam given by the second
step is maximized, n1 represents the refractive index of the optical
element 10 with respect to the first laser beam, n2 represents the
refractive index of the optical element 10 with respect to the second
laser beam, and n3 represents the refractive index of the optical element
10 with respect to the third laser beam.

[0083]The annular zone structure is configured such that the two or more
types of steps satisfy the following conditions (2), (3) and (4):

0.01<(E21-E11)/E11<0.10 (2)

0.04<(E31-E11)/E11<0.30 (3)

-100<φ1+φ2<-10 (4)

where E11=m11(λ1/(n1-1)),

E21=m21(λ2/(n2-1)),

E31=m31(λ3/(n3-1)),

φ1=ΣP12ih2i×m11 (unit: λ1),

φ2=ΣP22ih2i×m12 (unit: λ1),

[0084]P12i (i: integer) represents a 2i order coefficient of an
optical path difference function defining the first step, and P22i
represents a 2i order coefficient of an optical path difference function
defining the second step. That is, φ1 represents an additional
optical path length (unit: λ1) given by the first step in the
effective radius of a first area defined on the optical element 10 as an
area for converging the third laser beam on the record surface of the
optical disc D3, and φ2 represents an additional optical path length
(unit: λ1) given by the second step in the effective radius of
the first area.

[0085]E11 represents the amplitude of the diffraction effect given by the
annular zone structure to the first laser beam, E21 represents the
amplitude of the diffraction effect given by the annular zone structure
to the second laser beam, and E31 represents the amplitude of the
diffraction effect given by the annular zone structure to the third laser
beam. In this case, the diffraction effect relates particularly to the
effect of correcting the spherical aberration.

[0086]Each of the conditions (2) and (3) defines the diffraction effect of
the first step. The condition (2) means that the diffraction effect on
the second laser beam is larger than the diffraction effect on the first
laser beam. The condition (3) means that the diffraction effect on the
third laser beam is larger than the diffraction effect on the first laser
beam.

[0087]The objective optical system 30 satisfying the conditions (2) and
(3) is able to suitable correct the spherical aberration for each of the
first to third laser beams.

[0088]If (E21-E11)/E11 gets larger than or equal to the upper limit of the
condition (2), the spherical aberration becomes a overcorrected condition
particularly when the optical disc D2 is used; If (E21-E11)/E11 gets
smaller than or equal to the lower limit of the condition (2), the
spherical aberration becomes an undercorrected condition particularly
when the optical disc D2 is used.

[0089]If (E31-E11)/E11 gets larger than or equal to the upper limit of the
condition (3), the spherical aberration becomes a overcorrected condition
particularly when the optical disc D3 is used. If (E31-E11)/E11 gets
smaller than or equal to the lower limit of the condition (3), the
spherical aberration becomes an undercorrected condition particularly
when the optical disc D3 is used.

[0090]The condition (4) relates a sum of additional optical path lengths
given by the first and second steps. By satisfying the condition (4), the
relative spherical aberration caused when the optical disc being used is
switched between optical discs of different standards can be suppressed
more suitably. Further, it is possible to correct the spherical
aberration caused when the wavelength of the laser beam being used varies
by a minute amount. If (φ1+φ2) gets smaller than or equal to the
lower limit of the condition (4), the spherical aberration is brought to
an overcorrected condition when wavelength variations occur. If
(φ1+φ2) gets larger than or equal to the upper limit of the
condition (4), the spherical aberration is brought to an undercorrected
condition when wavelength variations occur.

[0091]If (φ1+φ2) gets larger than or equal to the upper limit of
the condition (4), the spherical aberration becomes an overcorrected
condition particularly when the optical disc D2 is used. If
(φ1+φ2) gets smaller than or equal to the lower limit of the
condition (4), the spherical aberration becomes an undercorrected
condition particularly when the optical disc D2 is used.

[0092]The objective optical system 30 may be configured to further satisfy
the following conditions (5) and (6).

0.015<(E21-E11)/E11<0.055 (5)

-75<φ1+φ2<-35 (6)

[0093]By satisfying the condition (5) and (6), it becomes possible to
achieve the compatibility with the three types of optical discs D1 to D3
in a higher degree of accuracy while decreasing change of the spherical
aberration caused when the wavelength of the laser beam used for
information recording or information reproducing varies in an minute
amount and change of the spherical aberration caused when the objective
lens made of resin is used.

[0094]Regarding the above described conditions (2), (3) and (4), it is
possible to express that the annular zone structure satisfies the
following conditions (7), (8) and (9):

0.01<(EP21-EP11)/EP11<0.10 (7);

0.04<(EP31-EP11)/EP11<0.30 (8); and

-100<Σ(ΔOPD11/λ1)+Σ(ΔOPD12/λ.s-
ub.1)<-10 (9),

where EP11=INT((ΔOPD11/λ1)+0.5)×(λ1(n1-1-
)),

EP21=INT((ΔOPD21/λ2)+0.5)×(λ2(n1-1)),

EP31=INT((ΔOPD31/λ3)+0.5)×(λ3(n3-1)),

[0095]In the above conditions, ΔOPD11/λ1 denotes an
optical path length difference given by the first step to the first laser
beam, ΔOPD21/λ2 denotes an optical path length
difference given by the first step to the second laser beam, and
ΔOPD31/λ3 denotes an optical path length difference
given by the first step to the third laser beam, and
ΔOPD12/λ1 denotes an optical path length difference
given by the second step to the first laser beam,

[0096]The conditions (7), (8) and (9) correspond to the conditions (2),
(3) and (4), respectively. Therefore, If (EP21-EP11)/EP11 gets larger
than or equal to the upper limit of the condition (7), the spherical
aberration becomes an overcorrected condition particularly when the
optical disc D2 is used. If (EP21-EP11)/EP11 gets smaller than or equal
to the lower limit of the condition (7), the spherical aberration becomes
an undercorrected condition particularly when the optical disc D2 is
used. If (EP31-EP11)/EP11 gets larger than or equal to the upper limit of
the condition (8), the spherical aberration becomes an overcorrected
condition particularly when the optical disc D3 is used. If
(EP31-EP11)/E11 gets smaller than or equal to the lower limit of the
condition (8), the spherical aberration becomes an undercorrected
condition particularly when the optical disc D3 is used. If
(Σ(ΔOPD11/λ1)+Σ(ΔOPD12/λ1))
gets smaller than the lower limit of the condition (9), the spherical
aberration is brought to an overcorrected condition when wavelength
variations occur. If
(Σ(ΔOPD11/λ1)+Σ(ΔOPD12/λ1))
gets larger than the upper limit of the condition (9), the spherical
aberration is brought to an undercorrected condition when wavelength
variations occur.

[0097]When the conditions (2), (3) and (4) are respectively expressed by
the conditions (7), (8), and (9), the conditions (5) and (6) can also be
respectively expressed by the following conditions (10) and (11).

0.015<(EP21-EP11)/EP11<0.055 (10)

-75<Σ(ΔOPD11/λ1)+Σ(ΔOPD12/λ.su-
b.1)<-35 (11)

[0098]One of the two or more types of steps in the annular zone structure
is configured such that the optical path length difference ΔOPD11
given to the first laser beam satisfies the following condition (12).

9.85<|ΔOPD11/λ1|<10.35 (12)

[0099]If an optical element not satisfying the condition (12) is used, it
becomes impossible to secure a sufficient level of use efficiency of
light for the first laser beam. Therefore, such an optical element is not
suitable for information recording or information reproducing for the
optical D1 with high accuracy.

[0100]A design example of the optical element 10 whose material satisfies
the condition (1) and which is configured to satisfy the conditions (2)
to (12) is m11=10, m21=6 and m31=5.

[0101]If the annular zone structure includes three types of steps, the
remaining one of the three types of steps (other than the first and the
second steps) may be configured such that the diffraction order at which
the diffraction efficiency for the first laser beam is maximized is the
second order, the diffraction order at which the diffraction efficiency
for the second laser beam is maximized is the first order, and the
diffraction order at which the diffraction efficiency for the first laser
beam is maximized is the first order. By this structure, even if the
wavelength of the laser beam being used varies by a minute amount, it is
possible to maintain the use efficiency of light at a high level while
suppressing the change amount of the spherical aberration to a low level.

[0102]If the annular zone structure of the optical element 10 includes
three types of steps, the remaining one of the three types of steps
(other than the first and second steps) may be designed such that an
absolute value of an optical path length difference given to the first
laser beam is approximately equal to 2λ1. By this structure,
even if the wavelength of the laser beam being used varies by a minute
amount, it is possible to maintain the use efficiency of light at a high
level while suppressing the change amount of the spherical aberration to
a low level.

[0103]By forming the above described annular zone structure within an area
(the first area) for converging the third laser beam on the record
surface of the optical disc D3 (i.e., an area contributing to convergence
of all of the first to third laser beams), a sufficient optical property
can be achieved.

[0104]It is also possible to form, within a second area located outside
the first area, an annular zone structure different from the annular zone
structure in the first area. If the second area is provided on the
optical element 10, the annular zone structure in the second area is
configured to contribute to convergence of each the first and second
laser beams on the record surface of the corresponding one of the optical
discs D1 and D2, and not to contribute to convergence of the third laser
beam on the optical disc D3. That is, the second area functions as an
aperture stop for the third laser beam.

[0105]The annular zone structure in the second area includes at least a
single type of step giving a certain optical path length difference to
the incident laser beam. In other words, the annular zone structure in
the second area is defined by a single type of optical path difference
function or by combination of a plurality of types of optical path
difference functions.

[0106]To achieve the function as the aperture stop, the annular zone
structure in the second area is configured such that the absolute value
of the optical path length difference given to the first laser beam by
the step in the second area is approximately equal to an odd multiple of
the wavelength of the first laser beam.

[0107]For example, an annular zone structure including a step giving an
optical path length difference, an absolute value of which is
approximately equal to 3λ1 or 5λ1 to the incident
laser beam is formed in the second area. When the third laser beam is
incident on such an annular zone structure formed in the second area, the
first order diffraction light and the second order diffraction light are
produced for the third laser beam. Therefore, the third laser beam passed
through the second area does not suitably converge on the record surface
of the optical disc D3.

[0108]It is also possible to form, within a third area located outside the
second area, an annular zone structure different from the annular zone
structures in the first and second areas. If the third area is provided
on the optical element 10, the annular zone structure in the third area
is configured to contribute to convergence of only each the first laser
beam on the record surface of the optical disc D1, and not to contribute
to convergence of each of the second and third laser beams. That is, the
third area is an area provided exclusively for the first laser beam to
secure the NA required for information recording or the information
reproducing for the optical disc D1 having the highest recording density.

[0109]The annular zone structure in the third area includes at least a
single type of step giving a certain optical path length difference to
the incident laser beam. To provide the third area with a function as an
aperture stop with respect to the second and third laser beams, the
annular zone structure in the third area is configured such that the
absolute value of the optical path length difference given by the annular
zone structure in the third area is different from the absolute value of
the optical path length difference given by the annular zone structure in
the second area. More specifically, at least one type of the plurality of
types of optical path difference functions defining the annular zone
structure in the third area is not equal to all of the optical path
difference functions defining the annular zone structure in the second
area.

[0110]For example, an annular zone structure giving an optical path length
difference, an absolute value of which is approximately equal to
λ1 to the incident laser beam is formed in the third area. By
this structure, it is possible to achieve the high diffraction efficiency
only for the first laser beam, and to suppress change of the spherical
aberration due to minute wavelength variations.

[0111]FIGS. 10A and 10B are conceptual illustrations of the annular zone
structure formed on the first surface 11 of the optical element 10. FIG.
10A is a front view illustrating the annular zone structure formed on the
first surface 11 of the optical element 10, and FIG. 10B is a cross
sectional view of the optical element 10 illustrating the annular zone
structure formed on the first surface 11 of the optical element 10. In
each of FIGS. 10A and 10B, the first to third areas are illustrated.

[0112]Hereafter, four numerical examples (first to fourth examples) of the
optical information recording/reproducing device 100 are described. In
the following examples, the protective layer thicknesses of the optical
discs D1-D3 are t1=0.1 mm, t2=0.6 mm and t3=1.2 mm.

FIRST EXAMPLE

[0113]The objective optical system 30 provided in the optical information
recording/reproducing device 100 according to a first example is shown in
FIG. 2. In the following, the explanation of the configuration of the
optical information recording/reproducing device 100 focuses on the
numerical configuration of the objective optical system 30 to clarify the
features of each example.

[0114]The following Table 1 shows concrete specifications of the objective
optical system 30 of the objective optical system 100 according to the
first example.

[0115]As indicated by the "Magnification" in Table 1, each of the first to
third laser beams is incident upon the objective optical system 30 as a
collimated beam. With this configuration, it is possible to prevent the
off-axis aberration from occurring during the tracking operation.

[0116]Table 2 shows a specific numerical configuration defined when the
optical disc D1 is used in the optical information recording/reproducing
device 100 provided with the objective optical system 30 shown in Table
1. The following Table 3 shows specific numerical configuration defined
when the optical disc D2 is used in the optical information
recording/reproducing device 100 provided with the objective optical
system 30 shown in Table 1. The following Table 4 shows specific
numerical configuration defined when the optical disc D3 is used in the
optical information recording/reproducing device 100 provided with the
objective optical system 30 shown in Table 1.

[0117]In the Tables 2-4, the surface #0 represents a light source (1A-1C),
the surfaces #1 and #2 represent the first and second surfaces 11 and 12
of the optical element 10, respectively, the surfaces #3 and #4 represent
the first and second surfaces 21 and 22 of the objective lens 20, and the
surfaces #5 and #6 represent the protective layer and the record surface
of the corresponding optical disc.

[0118]In Tables 18-20 (and in the following similar Tables), "r" denotes
the curvature radius (mm) of each optical surface, and "d" denotes the
thickness of an optical component or the distance (mm) from each optical
surface to the next optical surface during the information
reproduction/recordation.

[0119]Each of the first surface 11 (surface #1) of the optical element 10
and the first and second surfaces 21 and 22 (surfaces #3 and #4) of the
objective lens 20 is an aspherical surface. The following Table 5 shows
the cone constants K and aspherical coefficients A2i specifying the
shape of each of the first surface 11 (surface #1) of the optical element
10 and the first and second surfaces 21 and 22 (surfaces #3 and #4) of
the objective lens 20. In Table 5 (and in the following similar Tables),
the notation "E" means the power of 10 with an exponent specified by the
number to the right of E (e.g. "E-04" means "×10-4").

[0120]In this example, the first surface 11 of the optical element 10
includes the first area including the optical axis of the optical element
10, the second area formed outside the first area, and the third area
(i.e., the outermost area) formed outside the second area. The range with
which each of the first to third areas is formed can be expressed as
follows by a height h from the optical axis (i.e., by an effective
radius).

First Area: 0.000≦h≦1.230

Second Area: 1.230<h≦1.640

Third Area: 1.640<h≦2.125

[0121]The first area is configured as a common area contributing to
convergence of each of the first to third laser beams. The second area is
configured to contribute to convergence of each of the first and second
laser beams and not to contribute convergence of the third laser beam.
That is, the second area functions as an aperture stop for the third
laser beam.

[0122]The third area is an area for securing the NA required for
information recording/reproducing for the optical disc D1. More
specifically, the third area is configured to contribute to convergence
of the first laser beam and not to contribute to convergence of each of
the second and third laser beams. That is, the third area functions as an
aperture stop for the second and third laser beams.

[0123]To give the above described different types of functions to the
first to third areas, respectively, each of the first to third areas is
designed independently to have a unique annular zone structure. More
specifically, each of the first and second areas has the annular zone
structure defined by two types of optical path difference functions.

[0124]Table 6 shows the coefficients P2i of the optical path
difference function defining the annular zone structure of each of the
first to third areas on the first surface 11 of the optical element 10.
Table 7 shows the diffraction order m and an effective radius (height
from the optical axis) for each of the first to third areas. In Tables 6
and 7 (and in the following similar tables), "OPDF" means an optical path
difference function.

[0125]As shown in Tables 6 and 7, the annular zone structure in the first
area of the first surface 11 is configured by combining the two types of
optical path difference functions (1st and 2nd OPDFs) different
from each other. The annular zone structure in the second area of the
first surface 11 is configured by combining the two types of optical path
difference functions (3rd and 4th OPDFs) different from each
other. The annular zone structure in the third area of the first surface
11 is defined by the 5th optical path difference function.

[0126]It should be noted that although in this example the second and
tired areas are provided with the annular zone structures, the optical
element 10 is able to achieve an adequate optical property when the
annular zone structure is formed at least in the first area functions as
a common area for the first to third laser beams.

[0127]The following Table 8 shows a concrete configuration of the annular
zone structure formed in the first area of the optical element 10. In
Table 8 (and in the following similar tables), "No." denotes a number of
each annular zone counted from the optical axis, "hmin" and "hmax" denote
the range of each annular zone (heights from the optical axis),
ΔOPD11/λ1 denotes an optical path length difference
given by a first step (one of the two types of steps) to the first laser
beam, ΔOPD21/λ2 denotes an optical path length
difference given by the first step to the second laser beam,
ΔOPD31/λ3 denotes an optical path length difference
given by the first step to the third laser beam,
ΔOPD12/λ1 denotes an optical path length difference
given by a second step (the other of the two types of steps) to the first
laser beam, ΔOPD22/λ2 denotes an optical path length
difference given by the second step to the second laser beam, and
ΔOPD32/λ3 denotes an optical path length difference
given by the second step to the third laser beam.

[0128]The objective optical system 30 provided in the optical information
recording/reproducing device 100 according to a second example is shown
in FIG. 3. The following Table 9 shows concrete specifications of the
objective optical system 30 of the objective optical system 100 according
to the second example.

[0129]As indicated by the "Magnification" in Table 9, each of the first to
third laser beams is incident upon the objective optical system 30 as a
collimated beam.

[0130]Table 10 shows a specific numerical configuration defined when the
optical disc D1 is used in the optical information recording/reproducing
device 100 provided with the objective optical system 30 shown in Table
9. The following Table 11 shows specific numerical configuration defined
when the optical disc D2 is used in the optical information
recording/reproducing device 100 provided with the objective optical
system 30 shown in Table 9. The following Table 12 shows specific
numerical configuration defined when the optical disc D3 is used in the
optical information recording/reproducing device 100 provided with the
objective optical system 30 shown in Table 9.

[0131]In the Tables 10-12, the surface #0 represents a light source
(1A-1C), the surfaces #1 and #2 represent the first and second surfaces
11 and 12 of the optics element 10, respectively, the surfaces #3 and #4
represent the first and second surfaces 21 and 22 of the objective lens
20, and the surfaces #5 and #6 represent the protective layer and the
record surface of the corresponding optical disc.

[0132]Each of the first surface 11 (surface #1) of the optical element 10
and the first and second surfaces 21 and 22 (surfaces #3 and #4) of the
objective lens 20 is an aspherical surface. The following Table 13 shows
the cone constants K and aspherical coefficients A2i specifying the
shape of each of the first surface 11 (surface #1) of the optical element
10 and the first and second surfaces 21 and 22 (surfaces #3 and #4) of
the objective lens 20.

[0133]In this example, the first surface 11 of the optical element 10
includes the first area including the optical axis of the optical element
10, the second area formed outside the first area, and the third area
(i.e., the outermost area) formed outside the second area. The range with
which each of the first to third areas is formed can be expressed as
follows by a height h from the optical axis (i.e., by an effective
radius).

First Area: 0.000≦h≦1.230

Second Area: 1.230<h≦1.690

Third Area: 1.690<h≦2.125

[0134]The first to third areas of the second example respectively have the
same functions as those of the first to third areas of the first example.
In addition, in this example, each of the first and second areas has the
function of suppressing change of the spherical aberration caused when
the wavelength of the laser beam being used varies by a minute amount.

[0135]Table 14 shows the coefficients P2i of the optical path
difference function defining the annular zone structure of each of the
first to third areas on the first surface 11 of the optical element 10.
Table 15 shows the diffraction order m and an effective radius (height
from the optical axis) for each of the first to third areas.

[0136]As shown in Tables 14 and 15, the annular zone structure in the
first area of the first surface 11 is configured by combining the three
types of optical path difference functions (1st to 3rd OPDFs)
different from each other. The annular zone structure in the second area
of the first surface 11 is configured by combining the two types of
optical path difference functions (4th and 5th OPDFs) different
from each other. The annular zone structure in the third area of the
first surface 11 is defined by the 6th optical path difference
function.

[0137]The following Table 16 shows a concrete configuration of the annular
zone structure formed in the first area of the optical element 10. In
Table 16, ΔOPD11/λ1 denotes an optical path length
difference given by a first step (one of the three types of steps) to the
first laser beam, ΔOPD21/λ2 denotes an optical path
length difference given by the first step to the second laser beam,
ΔOPD31/λ3 denotes an optical path length difference
given by the first step to the third laser beam,
ΔOPD12/λ1 denotes an optical path length difference
given by a second step (a second type of the three types of steps) to the
first laser beam, ΔOPD22/λ2 denotes an optical path
length difference given by the second step to the second laser beam,
ΔOPD32/λ3 denotes an optical path length difference
given by the second step to the third laser beam,
ΔOPD13/λ1 denotes an optical path length difference
given by a third step (a third type of step of the three types of steps)
to the first laser beam, ΔOPD23/λ2 denotes an optical
path length difference given by the third step to the second laser beam,
and ΔOPD33/λ3 denotes an optical path length difference
given by the third step to the third laser beam.

[0138]The objective optical system 30 provided in the optical information
recording/reproducing device 100 according to a third example is shown in
FIG. 4. The following Table 17 shows concrete specifications of the
objective optical system 30 of the objective optical system 100 according
to the second example.

[0139]As indicated by the "Magnification" in Table 17, each of the first
to third laser beams is incident upon the objective optical system 30 as
a collimated beam.

[0140]Table 18 shows a specific numerical configuration defined when the
optical disc D1 is used in the optical information recording/reproducing
device 100 provided with the objective optical system 30 shown in Table
17. The following Table 19 shows specific numerical configuration defined
when the optical disc D2 is used in the optical information
recording/reproducing device 100 provided with the objective optical
system 30 shown in Table 17. The following Table 20 shows specific
numerical configuration defined when the optical disc D3 is used in the
optical information recording/reproducing device 100 provided with the
objective optical system 30 shown in Table 17.

[0141]In the Tables 18-20, the surface #0 represents a light source
(1A-1C), the surfaces #1 and #2 represent the first and second surfaces
11 and 12 of the optical element 10, respectively, the surfaces #3 and #4
represent the first and second surfaces 21 and 22 of the objective lens
20, and the surfaces #5 and #6 represent the protective layer and the
record surface of the corresponding optical disc.

[0142]Each of the first surface 11 (surface #1) of the optical element 10
and the first and second surfaces 21 and 22 (surfaces #3 and #4) of the
objective lens 20 is an aspherical surface. The following Table 21 shows
the cone constants K and aspherical coefficients A2i specifying the
shape of each of the first surface 11 (surface #1) of the optical element
10 and the first and second surfaces 21 and 22 (surfaces #3 and #4) of
the objective lens 20.

[0143]In this example, the first surface 11 of the optical element 10
includes the first area including the optical axis of the optical element
10, the second area formed outside the first area, and the third area
(i.e., the outermost area) formed outside the second area. The range with
which each of the first to third areas is formed can be expressed as
follows by a height h from the optical axis (i.e., by an effective
radius).

First Area: 0.000≦h≦1.250

Second Area: 1.250<h≦1.665

Third Area: 1.665<h≦2.295

[0144]The first to third areas of the third example respectively have the
same functions as those of the first to third areas of the first example.

[0145]Table 22 shows the coefficients P2i of the optical path
difference function defining the annular zone structure of each of the
first to third areas on the first surface 11 of the optical element 10.
Table 23 shows the diffraction order m and an effective radius (height
from the optical axis) for each of the first to third areas.

[0146]As shown in Tables 22 and 23, the annular zone structure in the
first area of the first surface 11 is configured by combining the two
types of optical path difference functions (1st and 2nd OPDFs)
different from each other. The annular zone structure in the second area
of the first surface 11 is configured by combining the two types of
optical path difference functions (3rd and 4th OPDFs) different
from each other. The annular zone structure in the third area of the
first surface 11 is defined by the 5th optical path difference
function.

[0147]The following Table 24 shows a concrete configuration of the annular
zone structure formed in the first area. In Table 24,
ΔOPD11/λ1 denotes an optical path length difference
given by a first step (one of the two types of steps) to the first laser
beam, ΔOPD21/λ2 denotes an optical path length
difference given by the first step to the second laser beam,
ΔOPD31/λ3 denotes an optical path length difference
given by the first step to the third laser beam,
ΔOPD12/λ1 denotes an optical path length difference
given by a second step (the other of the two types of steps) to the first
laser beam, ΔOPD22/λ2 denotes an optical path length
difference given by the second step to the second laser beam, and
ΔOPD32/λ3 denotes an optical path length difference
given by the second step to the third laser beam.

[0148]The objective optical system 30 provided in the optical information
recording/reproducing device 100 according to a third example is shown in
FIG. 5. The following Table 25 shows concrete specifications of the
objective optical system 30 of the objective optical system 100 according
to the second example.

[0149]As indicated by the "Magnification" in Table 25, each of the first
to third laser beams is incident upon the objective optical system 30 as
a collimated beam.

[0150]Table 26 shows a specific numerical configuration defined when the
optical disc D1 is used in the optical information recording/reproducing
device 100 provided with the objective optical system 30 shown in Table
25. The following Table 27 shows specific numerical configuration defined
when the optical disc D2 is used in the optical information
recording/reproducing device 100 provided with the objective optical
system 30 shown in Table 25. The following Table 28 shows specific
numerical configuration defined when the optical disc D3 is used in the
optical information recording/reproducing device 100 provided with the
objective optical system 30 shown in Table 25.

[0151]In the Tables 26-28, the surface #0 represents a light source
(1A-1C), the surfaces #1 and #2 represent the first and second surfaces
11 and 12 of the optical element 10, respectively, the surfaces #3 and #4
represent the first and second surfaces 21 and 22 of the objective lens
20, and the surfaces #5 and #6 represent the protective layer and the
record surface of the corresponding optical disc.

[0152]Each of the first surface 11 (surface #1) of the optical element 10
and the first and second surfaces 21 and 22 (surfaces #3 and #4) of the
objective lens 20 is an aspherical surface. The following Table 29 shows
the cone constants K and aspherical coefficients A2i specifying the
shape of each of the first surface 11 (surface #1) of the optical element
10 and the first and second surfaces 21 and 22 (surfaces #3 and #4) of
the objective lens 20.

[0153]In this example, the first surface 11 of the optical element 10
includes the first area including the optical axis of the optical element
10, the second area formed outside the first area, and the third area
(i.e., the outermost area) formed outside the second area. The range with
which each of the first to third areas is formed can be expressed as
follows by a height h from the optical axis (i.e., by an effective
radius).

First Area: 0.000≦h≦1.250

Second Area: 1.250<h≦1.665

Third Area: 1.665<h≦2.295

[0154]The first to third areas of the fourth example respectively have the
same functions as those of the first to third areas of the first example.

[0155]Table 30 shows the coefficients P2i of the optical path
difference function defining the annular zone structure of each of the
first to third areas on the first surface 11 of the optical element 10.
Table 31 shows the diffraction order m and an effective radius (height
from the optical axis) for each of the first to third areas.

[0156]As shown in Tables 30 and 31, the annular zone structure in the
first area of the first surface 11 is configured by combining the three
types of optical path difference functions (1st to 3rd OPDFs)
different from each other. The annular zone structure in the second area
of the first surface 11 is configured by combining the two types of
optical path difference functions (4th and 5th OPDFs) different
from each other. The annular zone structure in the third area of the
first surface 11 is defined by the 6th optical path difference
function.

[0157]The following Table 32 shows a concrete configuration of the annular
zone structure formed in the first area. In Table 32,
ΔOPD11/λ1 denotes an optical path length difference
given by a first step (one of the three types of steps) to the first
laser beam, ΔOPD21/λ2 denotes an optical path length
difference given by the first step to the second laser beam,
ΔOPD31/λ3 denotes an optical path length difference
given by the first step to the third laser beam,
ΔOPD12/λ1 denotes an optical path length difference
given by a second step (a second type of the three types of steps) to the
first laser beam, ΔOPD22/λ2 denotes an optical path
length difference given by the second step to the second laser beam,
ΔOPD32/λ3 denotes an optical path length difference
given by the second step to the third laser beam,
ΔOPD13/λ1 denotes an optical path length difference
given by a third step (a third type of step of the three types of steps)
to the first laser beam, ΔOPD23/λ2 denotes an optical
path length difference given by the third step to the second laser beam,
and ΔOPD33/λ3 denotes an optical path length difference
given by the third step to the third laser beam.

[0158]The following Table 33 shows values of the conditions of the above
described first to fourth examples. All of the first to fourth examples
satisfy at least the conditions (1), (2)-(4), (7)-(9) and (12).

[0159]By satisfying the condition (12), it is possible to secure high use
efficiency of light for each of the first to third laser beams. More
specifically, regarding the first example, the use efficiency of light
for the first laser beam is 71.4%, the use efficiency of light for the
second laser beam is 63.4%, and the use efficiency of light for the third
laser beam is 60.3%. Regarding the second example, the use efficiency of
light for the first laser beam is 83.3%, the use efficiency of light for
the second laser beam is 56.6%, and the use efficiency of light for the
third laser beam is 56.4%. Regarding the third example, the use
efficiency of light for the first laser beam is 86.8%, the use efficiency
of light for the second laser beam is 70.9%, and the use efficiency of
light for the third laser beam is 69.6%. Regarding the first example, the
use efficiency of light for the first laser beam is 70.3%, the use
efficiency of light for the second laser beam is 59.1%, and the use
efficiency of light for the third laser beam is 68.2%.

[0160]FIG. 6A is a graph illustrating the spherical aberration caused when
the first laser beam is used in the optical information
recording/reproducing device 100 having the objective optical system 30
according to the first example. FIG. 6B is a graph illustrating the
spherical aberration caused when the second laser beam is used in the
optical information recording/reproducing device 100 having the objective
optical system 30 according to the first example. FIG. 6C is a graph
illustrating the spherical aberration caused when the third laser beam is
used in the optical information recording/reproducing device 100 having
the objective optical system 30 according to the first example.

[0161]FIG. 7A is a graph illustrating the spherical aberration caused when
the first laser beam is used in the optical information
recording/reproducing device 100 having the objective optical system 30
according to the second example. FIG. 7B is a graph illustrating the
spherical aberration caused when the second laser beam is used in the
optical information recording/reproducing device 100 having the objective
optical system 30 according to the second example. FIG. 7C is a graph
illustrating the spherical aberration caused when the third laser beam is
used in the optical information recording/reproducing device 100 having
the objective optical system 30 according to the second example.

[0162]FIG. 8A is a graph illustrating the spherical aberration caused when
the first laser beam is used in the optical information
recording/reproducing device 100 having the objective optical system 30
according to the third example. FIG. 8B is a graph illustrating the
spherical aberration caused when the second laser beam is used in the
optical information recording/reproducing device 100 having the objective
optical system 30 according to the third example. FIG. 8C is a graph
illustrating the spherical aberration caused when the third laser beam is
used in the optical information recording/reproducing device 100 having
the objective optical system 30 according to the third example.

[0163]FIG. 9A is a graph illustrating the spherical aberration caused when
the first laser beam is used in the optical information
recording/reproducing device 100 having the objective optical system 30
according to the fourth example. FIG. 9B is a graph illustrating the
spherical aberration caused when the second laser beam is used in the
optical information recording/reproducing device 100 having the objective
optical system 30 according to the fourth example. FIG. 9C is a graph
illustrating the spherical aberration caused when the third laser beam is
used in the optical information recording/reproducing device 100 having
the objective optical system 30 according to the fourth example.

[0164]In each of FIGS. 6A-6C, 7A-7C, 8A-8C, and 9A-9C, a curve indicated
by a solid line represents the spherical aberration when the laser beam
having the design wavelength (shown in Tables 1, 9, 17, and 25) is
incident on the objective optical system 30, and a curve indicated by a
dashed line represents the spherical aberration when the wavelength of
the laser beam shifts by a 5 nm from the design wavelength.

[0165]As can be seen from FIGS. 6A-6C, 7A-7C, 8A-8C, and 9A-9C, each of
the first to fourth examples is able to suitably suppress the spherical
aberration for all of the optical discs D1-D3 when each of the first to
third laser beams is at the design wavelength.

[0166]As described above, the optical information recording/reproducing
device 100 is able to achieve the compatibility with the optical discs
D1-D3 with high accuracy while securing the high use efficiency of light.

[0167]As can be seen from FIGS. 7A-7C and FIGS. 9A-9C, each of the second
and fourth examples in which the annular zone structure in the first area
is formed by combining the three types of optical path difference
functions is able to suitably suppress change of the spherical aberration
when the wavelength variations occur. As can be seen from FIGS. 8A-8C,
the third example having the annular zone structure satisfying the
conditions (5), (6), (10) and (11) in the first area is able to suitably
suppress change of the spherical aberration when the wavelength
variations occur.

[0168]Although the present invention has been described in considerable
detail with reference to certain preferred embodiments thereof, other
embodiments are possible.

[0169]In the above described embodiment, the optical element 10 of the
objective optical system 30 is made of material having a high degree of
dispersion. However, the objective lens 20 may be made of material having
a high degree of dispersion if the objective optical system is configured
to suitable correct the chromatic aberration caused by employing the
material having a high degree of dispersion. Such a configuration for
suitably correcting the chromatic aberration is achieved by providing an
chromatic aberration correction element configured by cementing together
a pair of positive and negative lenses made of materials having different
degrees of dispersion, for the objective optical system.